Polyphase tectono-magmatic evolution during mantle exhumation in an ultra-distal, magma-poor rift domain: example of the fossil Platta ophiolite, SE Switzerland

Despite the fact that many studies have investigated mantle exhumation at ultra-slow-spreading ridges and magma-poor rifted margins, there are still numerous questions concerning the 3D architecture, magmatic, fluid, and thermal evolution of these domains that remain unexplained. Indeed, it has been observed in seismic data from ultra-distal magma-poor rifted margins that top basement is heavily structured and complex; however, the associated morpho-tectonic and magmatic processes remain ill constrained. The aim of this study is to describe the 3D top basement morphology, timing, and processes controlling the formation of an exhumed mantle domain preserved over about 200 km(2) in the Platta nappe in SE Switzerland. Detailed mapping of parts of the Platta nappe enabled to document the top basement architecture of an exhumed mantle domain, and to investigate its link to later, rift/oceanic structures, magmatic additions, and hydrothermal fluid systems. Our observations show: (1) a polyphase deformation history associated with mantle exhumation along exhumation faults overprinted by later high-angle normal faults, (2) a structured top basement morphology capped by magmato-sedimentary sequences, (3) a tectono-magmatic evolution that includes gabbros, emplaced at deeper levels and subsequently exhumed and overlain by younger extrusive magmatic additions, and (4) fluid systems related to serpentinization, calcification, hydrothermal vents, rodingitization, and spilitization affecting exhumed mantle and associated magmatic rocks. The overall observations provide new information on the temporal and spatial evolution of the tectonic and magmatic processes and their link to hydrothermal and sedimentary systems controlling the formation of ultra-distal, magma-poor rifted margins, and lithospheric breakup.